The present invention pertains to optical communications. Specifically, this invention relates to an electronic decoder/receiver which discovers the skew between N channels representing a single serial data stream and then corrects for that skew such that the original serial data stream can be reconstructed. In certain preferred embodiments, the decoder/receiver of the present invention is particularly suited for wave division multiplexing systems for fiber-optic data-communications and telecommunications. The present invention in one embodiment achieves the desired results with 8 parallel streams and the skew between the fastest and slowest channel is several bit times.
1. Description of the Problem the Invention Overcomes
As data rates have increased the useful distance for data transmission on multi-mode fiber has decreased (mostly due to modal dispersion problems). One technique to combat this problem is to divide a single data stream into N parallel streams that would be transmitted on N separate wavelengths of light on a single fiber at 1/Nth the original data rate. This wave division multiplexing (WDM) approach would result in a useful distance roughly N times the distance that was originally possible using a single serial stream.
However, in order to achieve this benefit, difficulties must be overcome associated with the fact that different wavelengths propagate at different speeds inside the fiber. Similarly, these different wavelengths may be launched at different power levels and may experience differing attenuation as they propagate through the optical system. This will result in significant variation of the signal level seen at the electronics on the receiver side. This variation will cause channels with high input levels to experience slightly less delay than channels with lower input levels, resulting in even more skew between channels (wavelengths). Moreover, since the intended application is to create a transceiver that is transparent to the end user, and since the transceiver is intended to operate with any protocol using the 8b10b code (ex. Gig-Ethernet and Fiber Channel), and since size, power and cost constraints make buffering undesirable, the skew discovery and compensation system must operate without inserting/deleting control/data blocks and without replacing any control/data blocks. Finally, this skew discovery/compensation must be accomplished in an environment where the skew between the slowest and the fastest wavelength may be several bit times.
The present invention overcomes all the difficulties described above.
2. Brief Comments Re Prior Art
Various deskewing techniques are known in the prior art, including systems requiring a training or sample signal. For example, U.S. Pat. No. 6,031,847 teaches a deskewing method that requires the use of a training sequence to measure skew between channels, as shown in FIG. 7 and required in claim 1 at column 23, lines 27-29. The use of a training sequence inherently slows the data transfer, and the use of the specially generated training sequence includes risks of contaminating the data streams. U.S. Pat. No. 6,079,035 requires the use of a sample signal (column 1, lines 38-42 and column 9, lines 6-11), with the same inherent limitations of the training sequence required by U.S. Pat. No. 6,031,847 discussed above. The present invention avoids the use of a training sequence and avoids the insertion or deletion of any control symbols relative to the original 8b10b data stream.
It is also known in the prior art to predict skew to create relative delays, as shown in U.S. Pat. No. 5,157,530. The prediction technique has serious inherent limitations requiring separate predictions as transmission conditions are changed. These limitations are overcome by the present invention which effectively measures the actual skew between channels using a phase detection technique. No predictions are required by the present invention; changes in transmission variables are automatically compensated for by the present invention.
The present invention utilizes known characteristics of the 8b10b code in conjunction with a phase detection technique using reference clock signals to actively measure relative bit skew between sets of adjacent channels. Since the phase detection technique will create ambiguities in determining whether a detected skew of a given channel bit is either ahead or behind of an adjacent channel bit, the present invention includes techniques for resolving such ambiguities. Resolution of ambiguity includes the use of two or more decoders, each programmed with different assumptions. Additionally, confirmation of the correctness of ambiguity resolution is achieved by checking alignment of known comma characters in each of the 8b10b data streams.
A primary object of the present invention is to enable data skew discovery and compensation for 8b10b code based communication systems without replacing any control or data blocks and without using training or sample signals. This makes the invention protocol independent as long as the protocols in question use the 8b10b code.
Another object of this invention is that it can compensate for skews up to 9-bits per WDM channel space. Thus an 8-channel WDM system (with 7 spaces between the wavelengths) could be constructed that would compensate for skews up to 63-bit times.
A further object of the present invention is to enable said skew discovery and compensation without inserting or deleting control or data blocks. This eliminates the need for complex buffering and packet monitoring schemes required to support the insertion and removal of control blocks to support skew measurement and compensation.
Another object of this invention is that it is insensitive to wavelength variations caused by temperature changes at the WDM transmitter.
A further object of this invention is that it will be un-affected by non-uniform channel separations in a WDM system. For example the spacing between WDM channel 0 and channel 1 may be 10-nanometers and the spacing between channel 1 and channel 2 may be 6-nanometers and the circuit will continue to operate properly.
Still another object of the present invention is that it introduces very little latency into the receive path since it operates at the bit/block level (as opposed to the frame level).
An additional object of this invention is that it will continue to operate correctly even in the presence of some skew unrelated to the chromatic skew for which it was designed to compensate.
Further objects and advantages will become apparent from the following description and drawings, wherein: